1. Field of the Invention
The present invention refers to a fluid pump, i.e. a pump for liquids and gases.
2. Description of Prior Art
It is known to use positive-displacement pumps for transporting fluids, said positive-displacement pumps consisting of a periodic displacer, a piston or a diaphragm, and two passive check valves. Due to the periodic movement of the piston or of the diaphragm, liquid is drawn into a pump chamber through the inlet valve and displaced from said pump chamber through the outlet valve. Due to the use of these valves, said known pumps are complicated and expensive. In addition, the direction of transport is predetermined by the arrangement of the valves. When the pumping direction of such an arrangement is to be reversed, such known pumps reuire a change of the operating direction of the valves from outside which entails a high expenditure. Such pumps are shown e.g. in Jarolav and Monika Ivantysyn; "Hydrostatische Pumpen und Motoren"; Vogel Buchverlag, Wurzburg, 1993.
Pumps of this type having a small constructional size and delivering small pumped streams are referred to as micropumps. The displacers of such pumps are typically implemented as a diaphragm, cf. P. Gravesen, J. Branebjerg, O. S. Jensen; Microfluidics--A review; Micro Mechanics Europe Neuchatel, 1993, pages 143-164. The displacers can be driven by different mechanisms. Piezoelectric drive mechanisms are shown in H. T. G. Van Lintel, F. C. M. Van de Pol. S. Bouwstra, A Piezoelectric Micropump Based on Micromachining of Silicon, Sensors & Actuators, 15, pages 153-167, 1988, S. Shoji, S. Nakagawa and M. Esashi, Micropump and sample injector for integrated chemical analyzing systems; Sensors and Actuators, A21-A23 (1990), pages 189-192, E. Stemme, G. Stemme; A valveless diffuser/nozzle based fluid pump; Sensors & Actuators A, 39 (1993) 159-167, and T. Gerlach, H. Wurmus; Working principle and performance of the dynamic micropump; Proc. MEMS'95; (1995), pages 221-226; Amsterdam, The Netherlands. Thermopneumatic mechanisms for driving the displacers are shown in F. C. M. Van de Pol, H. T. G. Van Lintel, M. Elwenspoek and J. H. J. Fluitman, A Termo-pneumatic Micropump Based on Micro-engineering Techniques, Sensors & Actuators, A21-A23, pages 198-202, 1990, B. Bustgens, W. Bacher, W. Menz, W. K. Schomburg; Micropump manufactured by thermoplastic molding; Proc. MEMS'94; (1994), pages 18-21. An electrostatic mechanism is shown in R. Zengerle, W. Geiger, M. Richter, J. Ulrich, S. Kluge, A. Richter; Application of Micro Diaphragm Pumps in Microfluid Systems; Proc. Actuator '94; 15.-17.6.1994; Bremen, Germany; pages 25-29. Furthermore, the displacers can be driven thermomechanically or magnetically.
As is also shown in the above-mentioned publications, either passive check valves or special flow nozzles can be used as valves, said check valves and said flow nozzles being both expensive and complicated. The direction of transport of micropumps can be reversed without forcibly controlling the valves, simply by effecting control at a frequency above the resonant frequency of said valves. In this context R. Zengerle, S. Kluge, M. Richter, A. Richter; A Bidirectional Silicon Micropump; Proc. MEMS '95; Amsterdam, Netherlands; pages 19-24, J. Ulrich, H. Fuller, R. Zengerle; Static and dynamic flow simulation through a KOH-etched micro valve; Proc. TRANSDUCERS '95, Stockholm, Sweden, (1995), pages 17-20, should be taken into account. The cause of this effect is a phase displacement between the movement of the displacer and the opening state of the valves. If the phase difference exceeds 90.degree., the opening state of the valves is anticyclic to their state in the normal forward mode and the pumping direction is reversed. A change of the operating direction of the valves from outside of the type required when macroscopic pumps are used can be dispensed with. The decisive phase difference between the displacer and the valves depends on the drive frequency of the pump on the one hand and on the resonant frequency of the movable valve member in the liquid surroundings on the other.
One disadvantage of this embodiment is to be seen in the fact that, upon constructing the valves, a compromise has to be found between the mechanical resonance in the liquid surroundings, the flow resistance, the fluidic capacity, i.e. the elastic volume deformation, the constructional size and the mechanical stability of these valves. It follows that these parameters, each of which may influence the pumping dynamics, cannot be ajusted to an optimum value independently of one another and part of them is opposed to a desired further miniaturization of the pump dimensions.
A general disadvantage entailed by the use of pumps with passive check valves is also the fact that, when switched off, the pumps do not block the medium to be transported. If the input pressure exceeds the output pressure by the pretension of the valves, the medium to be pumped will flow through the pump.
Micropumps using special flow nozzles have the disadvantage that they have a very low maximum pumping efficiency in the range of 10 to 20%.
DE-C 19534378.6 discloses a fluid pump comprising a pump body, a displacer and an elastic buffer. The displacer closes an inlet arranged in said pump body when occupying a first end position and leaves said inlet arranged in the pump body free when occupying a second end position. The known pump permits a net flow through an outlet which is also provided in the pump body. The buffer means bordering on the pump chamber formed by the displacer and the pump body makes the known fluid pump expensive and complicated.
Esashi, Shoji and Nakano describe in the article "Normally closed microvalve and micropump fabricated on a silicon wafer", Sensors and Actuators 20 (1989), pages 163-169, a gas microvalve which is normally closed. The valve consists of a glass plate having arranged therein a gas outlet opening which is adapted to be closed by means of a silicon-mesa structure that is provided with a valve seat and that is adapted to be operated by a piezoelectric drive. The silicon layer, in which the silicon-mesa structure is formed, and the glass plate additionally define a continuous channel between the gas outlet opening and a gas inlet opening formed in the silicon layer. The above-mentioned publication also describes a diaphragm-type micropump consisting of two one-way valves and a diaphragm with a piezoelectric drive means.